lib/core: Add a "break hardlink" API

This imports the code from rpm-ostree:
https://github.com/projectatomic/rpm-ostree/blob/9ff9f6c997d914cb7d97d6b59d8045ba64a1882c/src/libpriv/rpmostree-util.c#L742

I plan to use this for rofiles-fuse to implement
copyup: https://github.com/ostreedev/ostree/issues/1377

But it's just obviously generally useful for projects using
libostree I think.

Closes: #1378
Approved by: jlebon

src/libostree/ostree-core.c

@@ -749,6 +749,86 @@ ostree_content_file_parse (gboolean                compressed,
                                        cancellable, error);
 }
 
+/**
+ * ostree_break_hardlink:
+ * @dfd: Directory fd
+ * @path: Path relative to @dfd
+ * @skip_xattrs: Do not copy extended attributes
+ * @error: error
+ *
+ * In many cases using libostree, a program may need to "break"
+ * hardlinks by performing a copy.  For example, in order to
+ * logically append to a file.
+ *
+ * This function performs full copying, including e.g. extended
+ * attributes and permissions of both regular files and symbolic links.
+ *
+ * If the file is not hardlinked, this function does nothing and
+ * returns successfully.
+ *
+ * This function does not perform synchronization via `fsync()` or
+ * `fdatasync()`; the idea is this will commonly be done as part
+ * of an `ostree_repo_commit_transaction()`, which itself takes
+ * care of synchronization.
+ *
+ * Since: 2017.15
+ */
+gboolean ostree_break_hardlink (int               dfd,
+                                const char       *path,
+                                gboolean          skip_xattrs,
+                                GCancellable     *cancellable,
+                                GError          **error)
+{
+  struct stat stbuf;
+
+  if (!glnx_fstatat (dfd, path, &stbuf, AT_SYMLINK_NOFOLLOW, error))
+    return FALSE;
+
+  if (!S_ISLNK (stbuf.st_mode) && !S_ISREG (stbuf.st_mode))
+    return glnx_throw (error, "Unsupported type for entry '%s'", path);
+
+  const GLnxFileCopyFlags copyflags =
+    skip_xattrs ? GLNX_FILE_COPY_NOXATTRS : 0;
+
+  if (stbuf.st_nlink > 1)
+    {
+      guint count;
+      gboolean copy_success = FALSE;
+      char *path_tmp = glnx_strjoina (path, ".XXXXXX");
+
+      for (count = 0; count < 100; count++)
+        {
+          g_autoptr(GError) tmp_error = NULL;
+
+          glnx_gen_temp_name (path_tmp);
+
+          if (!glnx_file_copy_at (dfd, path, &stbuf, dfd, path_tmp, copyflags,
+                                  cancellable, &tmp_error))
+            {
+              if (g_error_matches (tmp_error, G_IO_ERROR, G_IO_ERROR_EXISTS))
+                continue;
+              g_propagate_error (error, g_steal_pointer (&tmp_error));
+              return FALSE;
+            }
+
+          copy_success = TRUE;
+          break;
+        }
+
+      if (!copy_success)
+        {
+          g_set_error (error, G_IO_ERROR, G_IO_ERROR_EXISTS,
+                       "Exceeded limit of %u file creation attempts", count);
+          return FALSE;
+        }
+
+      if (!glnx_renameat (dfd, path_tmp, dfd, path, error))
+        return FALSE;
+    }
+
+  return TRUE;
+}
+
 /**
  * ostree_checksum_file_from_input:
  * @file_info: File information